your smallest pipe is your network bottleneck. until your router becomes the smallest pipe, no need to upgrade-when your isp offers a +54mbs connection, then you could benefit from a bigger pipe on the router for internet access speed. even at 12mbs your only using 1/4 of your router pipe.
Only other reason to go with N:
If you have a bad wireless signal and want the range benefits with N, or have a need for greater than 54mbs between your home networked pc's (like large file transfers).
*Q6600@3330mhz 1.35v - True120 | Abit IP35pro | 2x2GB Gskil | 2 x EVGA GTS 250- shader 1890 | Corsair HX520 | OCZ Vertex 60GB/Seagate 7200.11 500GB
*x4 810@3250mhz 1.32v - Xigmatek S1283 | MSI K9A2 CF-F 790X | 2x1GB Kingston HyperX | 2 x MSI GTS 250- shader 1890 | Antec EW 500
*Q6600@3330mhz 1.36v - Scythe Mugen | Gigabyte GA-P35-DS3L | 2x1GB Ballistx | XFX GTX 260 (192) shader 1512 | OCZ Stealth 500w